21 



different, and Huxley has clearly pointed out the path for 

 investigation. It is probable that material already exists in 

 our museums for tracing the group through several important 

 steps in its development. We have already seen that the 

 modern procoelian type of this order goes back only to the 

 Upper Cretaceous, while the Behdonts, of our Triassic rocks 

 with their biconcave vertebras, are the oldest known Croco- 

 dilians. Our Jurassic, unfortunately, throws but little light on 

 the intermediate forms, but we know that the line was con- 

 tinued, as it was in the old world through Teleosaurus. The 

 beds of the Rocky Mountain Wealden have just furnished us 

 with a genuine "missing link," a saurian (Diplosaurus) with 

 essentially the skull and teeth of a modern Crocodile, and the 

 vertebras of its predecessor from the Trias. This peculiar rep- 

 tile clearly represents an important stage in the progressive 

 series, and evidently one soon after the separation of the Croc- 

 odile branch from the main stem. The modern Gavial type 

 appears to have been developed about the same time, as the form 

 was well established in the Upper Cretaceous genus, Thoraco- 

 saurus. The Teleosaurian group, with biconcave vertebrae, 

 evidently the parent stock of Crocodilians, became extinct with 

 Hyposaurus of the same horizon, leaving the Crocodile and 

 Gavial, with their more perfect procoelian vertebrae, to contend 

 for the supremacy. In the early Eocene, both of these types 

 were abundant, but some of the Crocodiles possessed characters 

 pointing towards the Alligators, which do not appear to have 

 been completely differentiated until later. 



Nothing is really known to-day of the earlier genealogy of 

 the Plerosauria, but our American forms, without teeth, are 

 clearly the last stage in their development before this peculiar 

 group became extinct. The oldest European form, Dimorphodon, 

 from the Lower Lias, had the entire jaws armed with teeth, and 

 was provided with a long tail. The later genus Pterodactylus 

 retained the teeth, but had essentially lost the tail ; while 

 Ramphorhynchus had retained the elongated tail, but had lost 

 the teeth from the fore part of both jaws. In the genus Pterano- 



